Alignment system and projection exposure apparatus

ABSTRACT

An alignment system for aligning a reticle having a pattern and an alignment mark with a photosensitive substrate to which the pattern of the reticle is to be transferred, is disclosed. The alignment system includes a movable substrate stage for holding the substrate, and a light-transparent plate having a mark for relative positioning with respect to at least one of the reticle and the substrate stage, wherein positioning of at least one of the reticle and the substrate stage is performed on the basis of the mark provided on the light-transparent plate.

FIELD OF THE INVENTION AND RELATED ART

[0001] This invention relates to an alignment system and a projectionexposure apparatus with the same. The present invention is suitablyusable in a lithographic process of manufacture of semiconductor devicesor other devices such as CCD or liquid crystal display, for example, forlithographically transferring or imagewise projecting a pattern of areticle or mask onto a substrate directly or through a projectionoptical system in step-and-repeat or step-and-scan method.

[0002] In projection exposure apparatuses for manufacture ofsemiconductor devices, for example, increases of density of anintegrated circuit have necessitated that a circuit pattern formed on areticle is projected and printed on a wafer (substrate) with higherresolution.

[0003] At the same time, due to miniaturization of a circuit pattern, itis required that a wafer and a reticle having an electronic circuitpattern formed thereon are aligned with each other very precisely.Generally, as an alignment method for the reticle and the wafer, thereis a baseline method wherein positional information about an alignmentmark provided on a wafer is detected (observed) through an alignmentmicroscope (alignment scope).

[0004] This method contains a factor for an error of reticle-to-waferalignment, called a baseline error which is an error related to baselinemeasurement.

[0005]FIG. 1A is a schematic view of a main portion of a conventionalprojection exposure apparatus, and FIG. 1B is a schematic view of aportion of FIG. 1A. The baseline measurement will be described briefly,with reference to these drawings.

[0006] In FIGS. 1A and 1B, a reticle 1 is held on a reticle stage 6 byattraction. The projection exposure apparatus is equipped with a reticlereference mark 19 which is positioned exactly with respect to aprojection optical system 7 and which is to be used for aligning thereticle 1 with respect to a predetermined position. Second markdetecting means 18 has a detection region at a predetermined positionwithin the projection field of the projection optical system 7, and itserves to optically detect, within this detection region, the positionalrelation between the reticle 1 and the reticle reference mark 19 as wellas the relative positional relation between a second reticle mark 5,provided on the reticle 1, and a second reference mark 14 formed on asubstrate stage 11. The second mark detecting means includes movingmeans. By use of this second mark detecting means 18 and with referenceto the reticle reference mark 19, a first reticle mark 4 provided on thereticle 1 is moved into registration with the reticle reference mark 19,and registration is measured. This is called first measurement. On thebasis of the result of measurement, a deviation between the reticle 1and the reticle reference mark 19 is detected.

[0007] A reference mark plate 12 is provided in a portion of the reticlestage 11, and it has formed thereon a first reference mark 13 which canbe detected through first mark detecting means 17 and a second referencemark 14 which can be detected through the second mark detecting means18. These first and second reference marks 13 and 14 are disposed with acertain interval corresponding to positions of the detecting regions ofthe first and second mark detecting means 17 and 18. The substrate stage11 is moved and positioned so that the second reticle mark 5 on thereticle 1 and the second reference mark 14 on the reference mark plate12 can be detected through the second mark detecting means 18. Aftersuch positioning, a relative positional deviation between the secondreference mark 14 and the detection center of the second reticle mark 5,and the deviation is memorized as a deviation of relative position ofthe reticle 1 and the substrate stage 11. This is called secondmeasurement.

[0008] Then, a deviation between the detection center of the first markdetecting means 17 and the first reference mark 13 on the referenceplate 12 is measured. This is called third measurement.

[0009] From the results of first to third measurements, the relativedistance between the reticle reference mark 19 and the detection centerof the first mark detecting means 17 is taken as a baseline, and arelative positional deviation detected by measurement is determined as abaseline correction value.

[0010] As described, in conventional projection exposure apparatuses,first the relative position between a first reticle mark 4 of a reticle1 and a reticle reference mark 19, positioned accurately with respect toa projection optical system 7 for alignment of the reticle 1 withrespect to a predetermined position, is detected (first measurement).Second, the relative position between a second reticle mark 5 of thereticle 1 and a second reference mark 14 formed on a reference markplate 12, provided in a portion of a substrate stage 11, is detected(second measurement). Third, the relative position between a firstreference mark 13, having predetected positional relation with the firstreference mark 14 on the reference mark plate 12, and the detectioncenter of first mark detecting means 17 capable of optically detecting amark on the substrate stage 11 and having its detection centerpositioned at a predetermined distance from the optical axis of theprojection optical system 7, is detected (third measurement). From theresults of these three detections, the distance between the detectioncenter of the first mark detecting means 17 and the reticle referencemark 19, for alignment of the reticle with a predetermined position, isdetected as a baseline, and it is memorized into a storing medium.

[0011] The baseline measurement in conventional projection exposureapparatuses is performed with the intervention of a reticle. Thiscreates a possibility that an error of reticle patterning causes abaseline error. It necessitates preparation of reticles to be usedexclusively with a particular projection exposure apparatus. This isvery inconvenient. In other words, performing baseline measurementthrough a peculiar reticle to be used with a particular projectionexposure apparatus, necessitates that the baseline measurement and stagecorrection measurement are executed after that reticle is loaded at apredetermined position in the exposure apparatus. This needs complicatedoperations for exposure apparatus control and for reticle control,numerous works have to be involved in exposure apparatus operation.

[0012] Further, the relative distance between the reticle reference mark19 and the detection center of the first mark detecting means 17 isdetected as baseline length, on the basis of three measurements made to(i) the reticle 1 and the reticle reference mark 19, (ii) the reticle 1and the substrate stage 11, and (iii) the substrate stage 11 and thedetection center of the first mark detecting means 17. Each of thesethree measurements may contain a measurement error. Therefore, there isa certain limitation to precision improvement in baseline measurement.Further, the necessity of three measurements produces a certainlimitation to improvement of baseline measurement speed.

[0013] Baseline measurement using a reticle peculiar to a particularexposure apparatus means that the baseline measurement is unattainableif a reticle loaded is different or when no reticle is loaded in theexposure apparatus. This provides a certain limitation to reduction ofoperation time for baseline measurement or to baseline measurementcontrol.

[0014] Furthermore, baseline measurement or stage running correctionmeasurement can not be performed if a reticle is placed at apredetermined position within the exposure apparatus. This applies anadverse effect to throughput of the whole exposure apparatus.

SUMMARY OF THE INVENTION

[0015] It is an object of the present invention to provide an improvedalignment system and/or an improved projection exposure apparatus bywhich a baseline, that is, the relative distance between a reticlereference mark exactly positioned with respect to a projection opticalsystem and a detection center of a first mark detecting means, fordetecting positional information related to a first reference markprovided on a substrate (wafer), can be measured with a baselinemeasurement error factor removed or reduced, such that simplification ofbaseline measurement control as well as improvement of baselinemeasurement precision and processing speed are assured, and such thatrelative alignment of the reticle and the substrate can be madeaccurately to assure high precision projection and transfer of a patternof the reticle onto the substrate.

[0016] It is another object of the present invention to provide animproved alignment system and/or an improved projection exposureapparatus by which, even if there is no reticle placed on a light pathwithin an exposure apparatus, a baseline, that is, the relative distancebetween a reticle reference mark exactly positioned with respect to aprojection optical system and a detection center of a first markdetecting means, for detecting positional information related to a firstreference mark provided on a substrate (wafer), can be measured with abaseline measurement error factor removed or reduced, such thatsimplification of baseline measurement control as well as improvement ofbaseline measurement precision and processing speed are assured, andsuch that relative alignment of the reticle and the substrate can bemade accurately to assure high precision projection and transfer of apattern of the reticle onto the substrate.

[0017] These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1A is a schematic view of a main portion of a conventionalprojection exposure apparatus with an alignment system.

[0019]FIG. 1B is a schematic view of a portion of FIG. 1A.

[0020]FIG. 2A is a schematic view of a main portion of a projectionexposure apparatus according to a first embodiment of the presentinvention.

[0021]FIG. 2B is a schematic view of a portion of FIG. 2A.

[0022]FIG. 3 is a schematic view for explaining reticle alignmentmeasurement, baseline measurement and substrate stage runningcorrection, in an exposure apparatus according to an embodiment of thepresent invention.

[0023]FIG. 4 is a schematic view of a main portion of a projectionexposure apparatus according to a second embodiment of the presentinvention.

[0024]FIG. 5 is a schematic view of a portion of FIG. 4.

[0025]FIG. 6 is a schematic view of a main portion of a projectionexposure apparatus according to a third embodiment of the presentinvention.

[0026]FIG. 7 is a schematic view of a portion of FIG. 6.

[0027]FIG. 8 is a flow chart of device manufacturing processes in anembodiment of the present invention.

[0028]FIG. 9 is a flow chart of a wafer process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029]FIG. 2A is a schematic view of a main portion of a projectionexposure apparatus according to a first embodiment of the presentinvention. FIG. 2B is a schematic view of a portion of FIG. 2A. In theexposure apparatus of FIGS. 2A and 2B, the clearance between a parallelflat glass 3 and a reticle 1 is actually very small, but forillustration and for ease in understanding, it is shown as large. Inthese drawings, there are two alignment systems whose components aredenoted by corresponding reference numerals.

[0030] In FIG. 2A, a reticle 1 has a pattern region (reticle patternregion) 2, in which a circuit pattern to be printed on a photosensitivesubstrate (wafer) W is formed, and a first reticle mark 4 to be used foralignment with respect to the parallel flat glass (parallel flat member)3 which is provided in accordance with the present embodiment. A reticlestage 6 holds the reticle 1 by attraction, and it has a structure fortwo-dimensional movement in X, Y and θ direction under control ofcontrolling means (drive control system), for controlling a drivingmechanism such as a motor, for example. Along an optical axis 8 betweenthe reticle 1 and a projection optical system 7, the parallel flat glass3 is fixed at a predetermined position with respect to the projectionoptical system 7, and it is positioned there.

[0031] The parallel flat glass 3 has marks formed thereon forpositioning of the reticle, a substrate stage and a photosensitivesubstrate. More specifically, on the parallel flat glass 3 there aresecond marks 10 as index marks to be used as a reference for alignmentof a first reticle mark 4 on the reticle 1, as well as first marks 9 asindex marks to be used as a reference for alignment of a secondreference mark 14 on a reference mark plate 12, provided in a portion ofa substrate stage 11. The first and second marks 9 and 10 are formed atpositions outside the incidence range of projection exposure light uponthe actual device pattern surface of the reticle 1. The second marks 10are provided symmetrically with respect to the center of the parallelflat glass 3. Thus, by these marks, the center of marks, that is, theoptical axis of the projection optical system 7 is determined. Thesubstrate stage 11 fixedly holds a photosensitive substrate W thereon,and it is provided with control means for controlling a drivingmechanism which serves to provide upward and downward motion of thephotosensitive substrate surface into registration with the image planeof the projection optical system 7, image plane defocus correctiondrive, and rotational drive of the photosensitive substrate for controlof alignment and yawing of the substrate.

[0032] Along two sides of the substrate stage 11, there are movablemirrors 16 which are fixedly provided to reflect beams from laserinterferometers 15. The beams emitted from the laser interferometers 15are perpendicular to the optical axis 8 of the projection optical system7. By means of these laser interferometers 15, the position and movementamount of the substrate stage 11 is measured continuously. The substratestage 11 has a reference mark plate 12 formed in a portion thereof. Foralignment of the substrate stage 11 with the parallel flat glass 3, thereference mark plate 12 is provided with a first reference mark 13 whichcan be detected through the first mark detecting means 17, and a secondreference marks 14 which can be detected through the second markdetecting means 18. The first and second reference marks 13 and 14 areprovided at a certain interval corresponding to the detection regions ofthe first and second mark detecting means 17 and 18.

[0033] The two-dimensional perpendicularity of the reference mark plate12 is held fixed so that it is registered, as much as possible, with thetwo-dimensional perpendicularity of the substrate stage 11. The firstmark detecting means 17 is outside the projection field of theprojection optical system 7, and it has a detection center at a positionspaced by a predetermined distance from the optical axis 8 of theprojection optical system 7. The first mark detecting means 17 serves asa detecting means for optically detecting marks provided on thephotosensitive substrate W or on both of the substrate stage 11 and thephotosensitive substrate W. The second mark detecting means 18 serves tooptically detect a relative positional relation of marks provided on thereticle 1, the parallel flat glass 3 and the substrate stage 11. It hasa detection region at a predetermined position within the projectionfield of the projection optical system 7. It includes moving means.

[0034]FIG. 2B is a schematic view for explaining reticle alignmentmeasurement, baseline measurement and substrate stage runningcorrection, in the exposure apparatus according to this embodiment ofthe present invention.

[0035] First, reticle alignment measurement in the exposure apparatuswill be explained in detail.

[0036] Initially, alignment illumination light AL emitted from analignment light source (not shown) goes along a light transmission path,having flexibility, and it is projected on the second mark 10 of theparallel flat glass 3 from the projection optical system 7 side. Theparallel flat glass 3 is positioned and mounted on the projectionoptical system 7 precisely. Detection illumination light AL passing thesecond alignment mark 10 illuminates the first reticle mark 4 on thereticle 1. Detection light passing the first reticle mark 4 goes througha detection optical system, and it is received by a camera which servesto convert a detection signal into an imagewise signal. An alignmentmark image 4 a thus taken is converted into a video signal. Then, imageprocessing is performed, and the amount of relative deviation betweenthe reticle 1 and the mark position of the parallel flat glass 3 isdetected. The thus detected deviation of the reticle 1 is then processedby a calculating means, and the result is stored into a storing medium.From the result of calculation processing made to the deviation of thereticle 1, the amount of corrective drive for the reticle is calculated,and the result is applied to the driving mechanism (second positioningmeans) of the reticle stage 6 on which the reticle is held byattraction. Thus, reticle stage correcting operation is performed,whereby the reticle position is corrected.

[0037] Here, the second mark detecting means 18 is positioned by use ofsecond positioning means so that the second mark 10 of the parallelglass plate 3 is registered, upon the processing picture image, with thefirst reticle mark 4, that is, the reticle and the parallel flat glassare aligned with each other.

[0038] Next, baseline measurement in the exposure apparatus of thisembodiment of the present invention will be described in detail.

[0039] In the exposure apparatus according to this embodiment of thepresent invention, the baseline length can be determined throughcalculation operation for results of two measurement operations.

[0040] A first measurement a measurement for detecting the relativepositional relation between the parallel flat glass 3 and the referencemark plate 12 on the substrate stage 11. Details of this measurement isas follows. The second mark detecting means 18 placed above the reticle1 and the substrate stage 11 are moved by a driving mechanism (firstpositioning means) to a predetermined station for baseline measurement,and focusing drive for the parallel flat glass 3 and the reference markplate 12 is performed. Then, by means of the second mark detecting means18, a relative positional deviation between the detection center of thefirst mark 9 on the parallel flat glass 3 and the second reference mark14 on the reference mark plate 12 is detected.

[0041] The thus detected deviation is memorized into a storing medium,as a relative positional deviation between the parallel flat glass 3 andthe reference mark plate 12. Here, the reticle 1 has been aligned withrespect to a predetermined position through the reticle alignmentmeasurement described hereinbefore, and for the baseline measurement thefirst mark 9 on the parallel flat glass 3 has been positioned below thetransmissive pattern of the reticle 1. Thus, detection light 23 from theabove of the reticle passes through the transmissive pattern of thereticle and it illuminates the first mark 9 on the parallel flat glass3.

[0042] A second measurement is a measurement for detecting the relativepositional relation between the reference mark plate 12 on the substratestage 11 and the detection center of the first mark detecting means 17.Details of this measurement is as follows. After completion of the firstmeasurement, the substrate stage 11 is moved in the optical axisdirection of the projection optical system 7 by a small amount, andfocusing of the first reference mark 13 of the reference mark plate 12and the detection center of the first mark detecting means 17 isperformed. Then, by using the first mark detecting means 17, a relativepositional deviation between the detection center of the first markdetecting means 17 and the first reference mark 13 on the reference markplate 12 is detected. The detected deviation is stored into a storingmedium.

[0043] The first measurement and second measurement are repeated bypredetermined times. Thereafter, calculation operation is performedwithin the storing medium, and a calculation value concerning therelative positional deviation between the detection center of the firstmark detecting means 17 and the first mark 9 of the parallel flat glass3 is produced. The thus obtained relative positional deviation Δ isstored into the storing medium, as a baseline error.

[0044] Here, the baseline error corresponds to a relative differentialdistance between the center of detection of the first mark 9 and a scopereference mark defined within the first mark detecting means 17.

[0045] For the baseline measurement, running correction of the substratestage 11 with respect to the parallel flat glass 3 has to be completedbeforehand. The running correction for the substrate stage 11 will bedescribed below, in detail.

[0046] Initially, the second mark detecting means 18 and the substratestage 11 are moved to a station where the second reference mark 14formed on the reference mark plate 12 and the first mark formed on theparallel flat glass 3 can be detected through the second mark detectingmeans 18. After the second mark detecting means 18 and the substratestage 11 are moved to the mark detection enabling position, a relativepositional deviation between these marks is measured. This measurementis repeated by predetermined times, and an average of running tiltdeviations of the substrate stage 11 with respect to the parallel flatglass 3 is calculated. The result is stored into a storing medium.

[0047] From the running tilt deviation of the substrate stage 11 withrespect to the parallel flat glass 3 and from the positional deviationof the reticle 1 with respect to the parallel flat glass 3, as detectedthrough the reticle alignment measurement, a running tilt of thesubstrate stage 11 with respect to the reticle 1 through the parallelflat glass 3 can be calculated. The value of calculation thus obtainedis put into the storing medium, and a correction value determined bycalculation is stored in a control storing medium as a offset forrunning deviation of the substrate stage 11.

[0048] Here, by using the second mark detecting means 18 and by usingthe first mark 9 of the parallel flat glass 3 and the second referencemark 14 of the substrate stage 11, relative alignment of the parallelflat glass 3 and the substrate stage 11 is accomplished. This is firstpositioning means.

[0049] An exposure apparatus with an alignment system according to thisembodiment of the present invention includes a control mechanism (firstpositioning means and second positioning means) for performingcorrective drive of the substrate stage 11 and the reticle 1 on thebasis of the results of measurement of positional deviations asdescribed above. Thus, the substrate stage 11 and the reticle 1 can bemoved through correction drive to appropriate positions, respectively.Namely, the reticle 1 can be aligned with respect to the parallel flatglass 3 by means of the reticle stage 6. With respect to the thuspositioned reticle 1, the wafer W whose alignment mark can be detectedby the first mark detecting means can be aligned while taking intoaccount the baseline error (and/or running deviation). By theseprocesses, each wafer shot position can be correctly positioned withrespect to the exposure area.

[0050] The parallel flat glass 3 to be used in the exposure apparatusmay be of integral type as shown in FIG. 2B, but alternatively, it maybe of separate structure such as shown in FIG. 3 wherein it comprises aparallel flat glass peripheral portion 20 having marks formed thereonand a parallel flat glass central portion 21 positioned at the opticalaxis of the projection optical system 7. When the parallel flat glass isseparated into a peripheral mark portion and a central optical axisportion, as above, the optical influence to an actual device patternportion to be exposed just below the projection optical system due toinsertion of a parallel flat glass member can be removed or reduced,through adjustment of the parallel flat glass central portion 21.Further, a small thermal deformation of the parallel flat glass to beproduced by absorption of illumination light by a mark on the parallelflat glass, can be reduced by separation of the exposure central portionand the mark portion. Therefore, any distortion of exposure pattern dueto a small thermal deformation of the parallel flat glass can beprevented.

[0051] In accordance with this embodiment of the present invention asdescribed above, a projection exposure apparatus may comprise a movablereticle stage for holding a reticle or photomask having a pattern to beexposed and plural alignment marks, a movable substrate stage forholding a photosensitive substrate onto which the pattern of the reticleis to be printed, a projection optical system for imagewise projectingthe pattern of the reticle in a projection region upon thephotosensitive substrate, and a parallel flat glass disposed between thereticle and the projection optical system and having marks forpositioning of the reticle, the substrate stage and the photosensitivesubstrate. With this arrangement, baseline measurement can be donewithout intervention of the reticle. As a result, an error of baselinemeasurement due to a reticle patterning error can be removed, such thatthe substrate can be aligned with respect to a predetermined positionvery precisely. The pattern of the reticle can then be projected andprinted on the wafer and, through subsequent development process,devices can be produced.

[0052]FIG. 4 is a schematic view of a main portion of a projectionexposure apparatus according to a second embodiment of the presentinvention. FIG. 5 is a schematic view of a portion of FIG. 4.

[0053] This embodiment differs from the first embodiment of FIG. 2A inthe following points. A first parallel flat glass 3 a (corresponding tothe glass 3 of FIG. 2A) having marks for positioning of a reticle 1, asubstrate stage 11 and a photosensitive substrate W, is provided on anoptical axis between the reticle and the projection optical system,while a second parallel flat glass with moving means is provided abovethe reticle and on the mark detection light path for the first parallelflat glass 3 a. With this arrangement, for baseline measurement, thebaseline can be measured without intervention of the reticle 1 andirrespective of whether a reticle is present across the mark detectionlight path. This enables that a baseline measurement error resultingfrom a reticle patterning error is removed and, additionally, thatbaseline measurement and substrate stage correction measurement areperformed even when no reticle is loaded in the exposure apparatus.Thus, the throughput of the whole exposure apparatus can be improved.The structure of the remainder is essentially the same as that of thefirst embodiment.

[0054] The structure of this embodiment will be described in detail,although it may be partially duplicate.

[0055] In the exposure apparatus of FIGS. 4 and 5, the clearance betweena parallel flat glass 3 and a reticle 1 is actually very small, but forillustration and for ease in understanding, it is shown as large. Inthese drawings, there are two alignment systems whose components aredenoted by corresponding reference numerals.

[0056] In FIG. 4, a reticle 1 has a pattern region (reticle patternregion) 2, in which a circuit pattern to be printed on a photosensitivesubstrate (wafer) W is formed, and a first reticle mark 4 to be used foralignment with respect to a first parallel flat glass 3 a which isprovided in accordance with this embodiment of the present embodiment. Areticle stage 6 holds the reticle 1 by attraction, and it has astructure for two-dimensional movement in X, Y and θ direction undercontrol of controlling means (drive control system), for controlling adriving mechanism such as a motor, for example. Along an optical axis 8between the reticle 1 and a projection optical system 7, the firstparallel flat glass 3 a is fixed at a predetermined position withrespect to the projection optical system 7, and it is positioned there.

[0057] The first parallel flat glass 3 a has marks formed thereon forpositioning of the reticle, a substrate stage and a photosensitivesubstrate. More specifically, on the first parallel flat glass 3 a thereare second marks 10 as index marks to be used as a reference foralignment of a first reticle mark 4 on the reticle 1, as well as firstmarks 9 as index marks to be used as a reference for alignment of asecond reference mark 14 on a reference mark plate 12, provided in aportion of a substrate stage 11.

[0058] The first and second marks 9 and 10 are formed at positionsoutside the incidence range of projection exposure light upon the actualdevice pattern surface of the reticle 1. The substrate stage 11 fixedlyholds a photosensitive substrate W thereon, and it is provided with adriving mechanism which serves to provide upward and downward motion ofthe photosensitive substrate surface into registration with the imageplane of the projection optical system 7, image plane defocus correctiondrive, and rotational drive of the photosensitive substrate for controlof alignment and yawing of the substrate.

[0059] Along two sides of the substrate stage 11, there are movablemirrors 16 which are fixedly provided to reflect beams from laserinterferometers 15. The beams emitted from the laser interferometers 15are perpendicular to the optical axis 8 of the projection optical system7. By means of these laser interferometers 15, the position and movementamount of the substrate stage 11 is measured continuously. The substratestage 11 has a reference mark plate 12 fixedly formed in a portionthereof. For alignment of the substrate stage 11 with the first parallelflat glass 3 a, the reference mark plate 12 is provided with a firstreference mark 13 which can be detected through the first mark detectingmeans 17, and a second reference marks 14 which can be detected throughthe second mark detecting means 18. The first and second reference marks13 and 14 are provided at a certain interval corresponding to thedetection regions of the first and second mark detecting means 17 and18.

[0060] The two-dimensional perpendicularity of the reference mark plate12 is held fixed so that it is registered, as much as possible, with thetwo-dimensional perpendicularity of the substrate stage 11. The firstmark detecting means 17 is outside the projection field of theprojection optical system 7, and it has a detection center at a positionspaced by a predetermined distance from the optical axis 8 of theprojection optical system 7. The first mark detecting means 17 serves asa detecting means for optically detecting marks provided on thephotosensitive substrate W or on both of the substrate stage 11 and thephotosensitive substrate W. The second mark detecting means 18 serves tooptically detect a relative positional relation of marks provided on thereticle 1, the first parallel flat glass 3 a and the substrate stage 11.It has a detection region at a predetermined position within theprojection field of the projection optical system 7. It includes movingmeans.

[0061] Additionally, there is a second parallel flat glass 3 b withmoving means, which is disposed above the reticle 1 and on the path ofdetection light 23 of the second mark detecting means. The secondparallel flat glass 3 b is inserted by a drive control system therefor,to a position above the reticle 1 and on the path of detection light 23of the second mark detecting means when the reticle 1 is not placed onthe path of detection light 23 of the second mark detecting means. Whenon the other hand the reticle 1 is placed on the path of detection light23 of the second mark detecting means, the second parallel flat glass ismoved out of the projection field and out of the path of detection light23 of the second mark detecting means, by means of the drive controlsystem for the second parallel flat glass 3 b.

[0062]FIG. 5 is a schematic view for explaining reticle alignmentmeasurement, baseline measurement and substrate stage runningcorrection, in the exposure apparatus according to this embodiment ofthe present invention.

[0063] First, reticle alignment measurement in the exposure apparatuswill be explained in detail.

[0064] Initially, alignment illumination light AL emitted from analignment light source (not shown) goes along a light transmission path,having flexibility, and it is projected on the second mark 10 of thefirst parallel flat glass 3 a from the projection optical system 7 side.The first parallel flat glass 3 a is positioned and mounted on theprojection optical system 7 precisely. Detection illumination light ALpassing the second alignment mark 10 illuminates the first reticle mark4 on the reticle 1. Detection light passing the first reticle mark 4goes through a detection optical system, and it is received by a camerawhich serves to convert a detection signal into an imagewise signal. Analignment mark image 4 a thus taken is converted into a video signal.Then, image processing is performed, and the amount of relativedeviation between the reticle 1 and the mark position of the firstparallel flat glass 3 a is detected. The thus detected deviation of thereticle 1 is then processed by a calculating means, and the result isstored into a storing medium. From the result of calculation processingmade to the deviation of the reticle 1, the amount of corrective drivefor the reticle is calculated, and the result is applied to the drivingmechanism (second positioning means) of the reticle stage 6 on which thereticle is held by attraction. Thus, reticle stage correcting operationis performed, whereby the reticle position is corrected.

[0065] Next, baseline measurement in the exposure apparatus of thisembodiment of the present invention will be described in detail.

[0066] In the exposure apparatus according to this embodiment of thepresent invention, the baseline length can be determined throughcalculation operation for results of two measurement operations.

[0067] A first measurement a measurement for detecting the relativepositional relation between the first parallel flat glass 3 a and thereference mark plate 12 on the substrate stage 11. Details of thismeasurement is as follows. When there is a reticle 1 on the path ofdetection light 23 of the second mark detecting means, the second markdetecting means 18 placed above the reticle 1 and the substrate stage 11are moved by a driving mechanism (first positioning means) to apredetermined station for baseline measurement, and focusing drive forthe parallel flat glass 3 a and the reference mark plate 12 isperformed. Then, by means of the second mark detecting means 18, arelative positional deviation between the detection center of the firstmark 9 on the first parallel flat glass 3 a and the second referencemark 14 on the reference mark plate 12 is detected.

[0068] The thus detected deviation is memorized into a storing medium,as a relative positional deviation between the first parallel flat glass3 a and the reference mark plate 12. Here, the reticle 1 has beenaligned with respect to a predetermined position through the reticlealignment measurement described hereinbefore, and for the baselinemeasurement the first mark 9 on the first parallel flat glass 3 a hasbeen positioned below the transmissive pattern of the reticle 1. Thus,detection light 23 from the above of the reticle passes through thetransmissive pattern of the reticle and it illuminates the first mark 9on the first parallel flat glass 3 a.

[0069] When there is no reticle 1 on the path of detection light 23 ofthe second mark detecting means, the second mark detecting means 18 andthe substrate stage 11 are moved to a predetermined station for baselinemeasurement, and focusing of the first parallel flat glass 3 a and thereference mark plate 12 is performed. By means of the second markdetecting means 18, a relative positional deviation between the firstmark 19 on the first parallel flat glass 3 a and the second referencemark 14 on the reference mark plate 12 is detected. The thus detecteddeviation is stored into a storing medium, as a relative positionaldeviation between the first parallel flat glass 3 a and the referencemark plate 12. Here, as the detection light 23 of the second markdetecting means passes the second parallel flat glass 3 b, any opticalinfluence to be produced as a result of not passing through the reticle1 can be removed or reduced.

[0070] Further, the first measurement described above may be performedby use of the arrangement of second mark detecting means 18 such asshown in FIG. 6. In accordance with the arrangement of second markdetecting means 18 shown in FIG. 6, a first lens 25 and a drive controlmeans 26 having a driving mechanism and a drive control system areprovided within the second mark detecting means 18. When no reticle ispresent on the path of detection light 23 of the second mark detectingmeans, the first lens is moved to a position effective to correct anyoptical influence to the detection light 23 of the second mark detectingmeans, due to absence of the reticle. When on the other hand a reticleis present on the path of detection light 23 of the second markdetecting means, the first lens is moved to a position effective tocorrect any optical influence to the detection light 23 of the secondmark detecting means due to presence of the reticle. With thisstructure, as a result, substantially the same optical correction effectas attainable with the exposure apparatus of FIG. 4 with a secondparallel flat glass 3 b, is accomplished.

[0071] A second measurement is a measurement for detecting the relativepositional relation between the reference mark plate 12 on the substratestage 11 and the detection center of the first mark detecting means 17.Details of this measurement is as follows. After completion of the firstmeasurement, the substrate stage 11 is moved in the optical axisdirection of the projection optical system 7 by a small amount, andfocusing of the first reference mark 13 of the reference mark plate 12and the detection center of the first mark detecting means 17 isperformed. Then, by using the first mark detecting means 17, a relativepositional deviation between the detection center of the first markdetecting means 17 and the first reference mark 13 on the reference markplate 12 is detected. The detected deviation is stored into a storingmedium.

[0072] The first measurement and second measurement are repeated bypredetermined times. Thereafter, calculation operation is performedwithin the storing medium, and a calculation value concerning therelative positional deviation between the detection center of the firstmark detecting means 17 and the first mark 9 of the first parallel flatglass 3 a is produced. The thus obtained relative positional deviation Δis stored into the storing medium, as a baseline error.

[0073] For the baseline measurement, running correction of the substratestage 11 with respect to the first parallel flat glass 3 a has to becompleted beforehand. The running correction for the substrate stage 11will be described below, in detail.

[0074] Initially, the second mark detecting means 18 and the substratestage 11 are moved to a station where the second reference mark 14formed on the reference mark plate 12 and the first mark 9 formed on thefirst parallel flat glass 3 a can be detected through the second markdetecting means 18. After the second mark detecting means 18 and thesubstrate stage 11 are moved to the mark detection enabling position, arelative positional deviation between these marks is measured. Thismeasurement is repeated by predetermined times, and an average ofrunning tilt deviations of the substrate stage 11 with respect to thefirst parallel flat glass 3 a is calculated. The result is stored into astoring medium.

[0075] From the running tilt deviation of the substrate stage 11 withrespect to the first parallel flat glass 3 a and from the positionaldeviation of the reticle 1 with respect to the first parallel flat glass3 a, as detected through the reticle alignment measurement, a runningtilt of the substrate stage 11 with respect to the reticle 1 through thefirst parallel flat glass 3 a can be calculated. The value ofcalculation thus obtained is put into the storing medium, and acorrection value determined by calculation is stored in a controlstoring medium as a offset for running deviation of the substrate stage11.

[0076] Here, the reticle 1 has been aligned with respect to apredetermined position through the reticle alignment measurementdescribed hereinbefore, and for the baseline measurement the first mark9 on the first parallel flat glass 3 a has been positioned below thetransmissive pattern of the reticle 1. Thus, detection light 23 from theabove of the reticle 1 passes through the transmissive pattern of thereticle 1 and it illuminates the first mark 9 on the first parallel flatglass 3 a.

[0077] When there is no reticle 1 on the path of detection light 23 ofthe second mark detecting means, similarly to the baseline measurementdescribed, the second parallel flat glass 3 b is moved to the path ofthe second mark detection light 23, so that the second mark detectionlight 23 goes through the second parallel flat glass 3 b. By this, anyoptical influence to be produced by not passing the reticle 1 can beremoved or reduced.

[0078] In a case where the exposure apparatus of FIG. 5 is equipped withsecond mark detecting means 18 of the structure of FIG. 6, when there isno reticle 1 on the path of detection light 23 of the second markdetecting means, the first lens is moved to a position effective tocorrect any optical influence to the detection light 23 of the secondmark detecting means, due to absence of the reticle. When on the otherhand a reticle is present on the path of detection light 23 of thesecond mark detecting means, the first lens is moved to a positioneffective to correct any optical influence to the detection light 23 ofthe second mark detecting means due to presence of the reticle. Withthis structure, as a result, substantially the same optical correctioneffect as attainable with the exposure apparatus with a second parallelflat glass 3 b, is accomplished.

[0079] The exposure apparatus according to this embodiment of thepresent invention is provided with a control mechanism for performingcorrective drive of the substrate stage and the reticle stage on thebasis of the results of these positional deviation measurements.Therefore, the substrate stage and the reticle stage can be moved bycorrective drive to appropriate positions.

[0080] It is to be noted that the exposure apparatus of FIG. 4 isequipped with a control mechanism for performing corrective drive of thesecond parallel flat glass 3 b to an appropriate position for opticalcorrection described above.

[0081] Also, the exposure apparatus of FIG. 6 is equipped with a controlmechanism for performing corrective drive of the first lens with movingmeans within the second mark detecting means to an appropriate positionfor optical correction described above.

[0082] The first parallel flat glass 3 a to be used in the exposureapparatus in accordance with the present invention may be of integraltype as shown in FIG. 5, but alternatively, it may be of separatestructure such as shown in FIG. 7 wherein it comprises a parallel flatglass peripheral portion 20 a having marks formed thereon and a parallelflat glass central portion 21 a positioned at the optical axis of theprojection optical system 7. When the first parallel flat glass 3 a isseparated into a peripheral mark portion and a central optical axisportion, as above, the optical influence to an actual device patternportion to be exposed just below the projection optical system due toinsertion of a parallel flat glass member can be removed or reduced,through adjustment of the central portion of the first parallel flatglass 3 a. Further, a small thermal deformation of the first parallelflat glass to be produced by absorption of illumination light by a markon the first parallel flat glass, can be reduced by separation of theexposure central portion and the mark portion. Therefore, any distortionof exposure pattern due to a small thermal deformation of the firstparallel flat glass 3 a can be prevented.

[0083] Next, an embodiment of device manufacturing method which uses aprojection exposure apparatus and/or an alignment system such asdescribed hereinbefore, will be explained.

[0084]FIG. 8 is a flow chart of procedure for manufacture ofmicrodevices such as semiconductor chips (e.g. ICs or LSIs), liquidcrystal panels, or CCDs, for example.

[0085] Step 1 is a design process for designing a circuit of asemiconductor device. Step 2 is a process for making a mask on the basisof the circuit pattern design. Step 3 is a process for preparing a waferby using a material such as silicon. Step 4 is a wafer process which iscalled a pre-process wherein, by using the so prepared mask and wafer,circuits are practically formed on the wafer through lithography. Step 5subsequent to this is an assembling step which is called a post-processwherein the wafer having been processed by step 4 is formed intosemiconductor chips. This step includes assembling (dicing and bonding)process and packaging (chip sealing) process. Step 6 is an inspectionstep wherein operation check, durability check and so on for thesemiconductor devices provided by step 5, are carried out. With theseprocesses, semiconductor devices are completed and they are shipped(step 7).

[0086]FIG. 9 is a flow chart showing details of the wafer process.

[0087] Step 11 is an oxidation process for oxidizing the surface of awafer. Step 12 is a CVD process for forming an insulating film on thewafer surface. Step 13 is an electrode forming process for formingelectrodes upon the wafer by vapor deposition. Step 14 is an ionimplanting process for implanting ions to the wafer. Step 15 is a resistprocess for applying a resist (photosensitive material) to the wafer.Step 16 is an exposure process for printing, by exposure, the circuitpattern of the mask on the wafer through the exposure apparatusdescribed above. Step 17 is a developing process for developing theexposed wafer. Step 18 is an etching process for removing portions otherthan the developed resist image. Step 19 is a resist separation processfor separating the resist material remaining on the wafer after beingsubjected to the etching process. By repeating these processes, circuitpatterns are superposedly formed on the wafer.

[0088] With these processes, high density microdevices can bemanufactured.

[0089] In accordance with the embodiments of the present invention asdescribed hereinbefore, the baseline between a reticle reference markexactly positioned with respect to a projection optical system and adetection center of a first mark detecting means, for detectingpositional information related to a first reference mark provided on asubstrate (wafer), can be measured with a baseline measurement errorfactor removed or reduced, such that simplification of baselinemeasurement control as well as improvement of baseline measurementprecision and processing speed are assured. Therefore, in an alignmentsystem and/or a projection exposure apparatus according to any of theseembodiments of the present invention, relative alignment of the reticleand the substrate can be made accurately, and high precision projectionand transfer of a pattern of the reticle onto the substrate is assured.

[0090] Further, even if there is no reticle on the path in an exposureapparatus, use of a second parallel flat glass enables accomplishment ofan alignment system and/or a projection exposure apparatus by whichsimplification of baseline measurement control as well as improvement ofbaseline measurement precision and processing speed are assured.

[0091] Additionally, since a reticle patterning error component and areticle measurement error component can be removed from a baselinemeasurement component, the baseline measurement can be performed withoutaffected by various reticle precisions. This is very effective toenhancement of baseline measurement precision.

[0092] As compared with conventional baseline measurement based oncalculation operation made to components of three types of measurements,in accordance with these embodiments of the present invention thebaseline measurement value is obtainable with calculation operation totwo measurement components. This is very effective to improvecalculation processing time, and also to reduce a measurement errorrelated to various baseline measurement components.

[0093] Further, the baseline measurement is performed by use of aparallel flat glass fixed to the apparatus, without relying on areticle, the necessity of a reference reticle as required inconventional baseline measurement is removed.

[0094] Furthermore, because the baseline measurement can be done withouta reticle, reticle replacement and baseline measurement can be performedin parallel. This is very effective to improve the throughput of theexposure apparatus.

[0095] In addition, use of an optically correcting mechanism forcorrecting optical influence due to absence of a reticle in an exposureapparatus, enables proper baseline measurement independently of whetherthere is a reticle loaded or not within the exposure apparatus.

[0096] While the invention has been described with reference to thestructures disclosed herein, it is not confined to the details set forthand this application is intended to cover such modifications or changesas may come within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. An alignment system for aligning a reticle havinga pattern and an alignment mark with a photosensitive substrate to whichthe pattern of the reticle is to be transferred, said alignment systemcomprising: a movable substrate stage for holding the substrate; and alight-transparent plate having a mark for relative positioning withrespect to at least one of the reticle and said substrate stage, whereinpositioning of at least one of the reticle and said substrate stage isperformed on the basis of the mark provided on said light-transparentplate.
 2. An alignment system according to claim 1, further comprising(i) first and second reference marks provided on said substrate stage,(ii) first mark detecting means for optically detecting a mark on thesubstrate without through any of said light-transparent plane and thesubstrate, and (iii) second mark detecting means for optically detectinga mark provided on at least one of the reticle, said substrate stage,said light-transparent plate and the substrate.
 3. An alignment systemaccording to claim 2, wherein marks of the reticle and saidlight-transparent plate can be detected through said second markdetecting means, and wherein on the basis of said detection the reticleand said light-transparent plate can be aligned with each other.
 4. Analignment system according to claim 2, wherein the second reference markand the mark of said light-transparent mark can be detected through saidsecond mark detecting means, wherein the first reference mark can bedetected through said first mark detecting means, and wherein, on thebasis of these detections, a relative positional deviation between adetection center of said first mark detecting means and the mark of saidlight-transparent plate can be detected.
 5. An alignment systemaccording to claim 2, wherein said second mark detecting means isoperable to move said second mark detecting means and said substratestage to a position where the mark of said light-transparent plate andthe second reference mark can be detected through said second markdetecting means, and wherein information related to running tilt of saidsubstrate stage with respect to said light-transparent plate can bedetected on the basis of detection of the second reference mark and themark of said light-transparent plate made through said second markdetecting means after the movement.
 6. An alignment system according toclaim 2, wherein said light-transparent plate comprises a separatestructure of a first portion for bearing a mark thereon and a secondportion for receiving exposure light.
 7. An alignment system accordingto claim 2, further comprising a second light-transparent plate of athickness corresponding to the thickness of the reticle, wherein saidsecond light-transparent plate is inserted to a path of detection lightof said second mark detecting means when the reticle is not loaded. 8.An alignment system according to claim 2, wherein said second markdetecting means include a correcting lens which is movable, when thereticle is not loaded, to a position for correcting an optical influencedue to absence of the reticle.
 9. An alignment system for aligning areticle having a pattern and an alignment mark with a photosensitivesubstrate to which the pattern of the reticle is to be transferred, saidalignment system comprising: a movable substrate stage for holding thesubstrate; and a light-transparent plate having a mark for relativepositioning with respect to at least one of the reticle and saidsubstrate stage; first and second reference marks provided on saidsubstrate stage; first mark detecting means for optically detecting amark on the substrate without through any of said light-transparentplate and the reticle; and second mark detecting means for opticallydetecting a mark provided on at least one of the reticle, said substratestage, said light-transparent plate and the substrate; wherein saidsecond mark detecting means is operable to detect relative positionalinformation between the mark of said light-transparent plate and thesecond reference mark, while said first mark detecting means is operableto detect relative positional information between the first referencemark and a detection center of said first mark detecting means, on thebasis of which relative positional information between the mark of saidlight-transparent plate and the detection center of said first markdetecting means can be detected; and wherein alignment between thereticle and the substrate can be performed on the basis of the relativepositional information between the mark of said light-transparent plateand the detection center of said first mark detecting means.
 10. Anexposure apparatus usable with a reticle having a pattern and analignment mark and a photosensitive substrate, for transferring thepattern of the reticle onto the substrate, said apparatus comprising: aprojection optical system for projecting the pattern of the reticle ontothe substrate; and a light-transparent plate provided between saidprojection optical system and a position where the reticle is to bedisposed, said light-transparent plate having an alignment mark to beused for relative alignment of the reticle and the substrate.
 11. Anapparatus according to claim 10, further comprising (i) a movablesubstrate stage for holding the substrate, (ii) first and secondreference marks provided on said substrate stage, (iii) first markdetecting means for optically detecting a mark on the substrate withoutthrough any of said light-transparent plate and the reticle, and (iv)second mark detecting means for optically detecting a mark provided onat least one of the reticle, said substrate stage, saidlight-transparent plate and the substrate, wherein said second markdetecting means is operable to detect relative positional informationbetween the mark of said light-transparent plate and the secondreference mark, while said first mark detecting means is operable todetect relative positional information between the first reference markand a detection center of said first mark detecting means, on the basisof which relative positional information between the mark of saidlight-transparent plate and the detection center of said first markdetecting means can be detected, and wherein alignment between thereticle and the substrate can be performed on the basis of the relativepositional information between the mark of said light-transparent plateand the detection center of said first mark detecting means.
 12. Adevice manufacturing method, comprising the steps of: detecting relativepositional information between a detection center of first markdetecting means and a first reference mark provided on a substrate stagefor holding a photosensitive substrate; detecting relative positionalinformation between a second reference mark provided on the substratestage and a mark provided on a light-transparent plate, disposed betweena projection optical system and a position where a reticle is to bedisposed; detecting relative positional information between the mark onthe light-transparent plate and a detection center of the first markdetecting means; aligning the reticle and the substrate on the basis ofthe relative positional information between the mark of thelight-transparent plate and the detection center of the first markdetecting means; transferring a pattern of the reticle onto thesubstrate, aligned with each other, through the projection opticalsystem.